cmd/ld: ignore .Linfo_stringNN variables in clang .o files

http://build.golang.org/log/c7a91b6eac8f8daa2bd17801be273e58403a15f2

# cmd/pprof
/linux-386-clang-9115aad1dc4a/go/pkg/linux_386/net.a(_all.o): sym#16: ignoring .Linfo_string0 in section 16 (type 0)
/linux-386-clang-9115aad1dc4a/go/pkg/linux_386/net.a(_all.o): sym#17: ignoring .Linfo_string1 in section 16 (type 0)
/linux-386-clang-9115aad1dc4a/go/pkg/linux_386/net.a(_all.o): sym#18: ignoring .Linfo_string2 in section 16 (type 0)
/linux-386-clang-9115aad1dc4a/go/pkg/linux_386/net.a(_all.o): sym#20: ignoring .Linfo_string0 in section 16 (type 0)
/linux-386-clang-9115aad1dc4a/go/pkg/linux_386/net.a(_all.o): sym#21: ignoring .Linfo_string1 in section 16 (type 0)
...

I don't know what these are. Let's ignore them and see if we get any further.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/155030043